1. Field of the Invention
The present invention relates to a wireless transmission/reception system capable of receiving wireless transmission signals from a plurality of wireless transmitters at the same time and recovering an information code series of each wireless transmitter through demodulation, the information code series being generated at each wireless transmitter through modulation.
2. Description of Related Art
A wireless transmission system is known in which a flow (stream) of information of integrated moving images and audio signals capable of being reproduced synchronously is transmitted via radio waves. As a typical wireless transmission system, a wireless camera for shooting and transmitting live scenes of sports, events or the like to a receiver at a base station and a program distribution system which transmits via a wireless LAN have already been realized. A wireless camera is high in mobility due to cordless and is suitable for a sub-camera or news reporting equipment moving in a broad area. To deal with recent digitalization of broadcasting, the transmitting system for transmitting from a wireless camera to a base station is shifting from a conventional system adopting analog modulation as a transmission system to a new system adopting digital modulation. Such a recent digitalization of wireless transmission system allows error correction technologies to be adopted, and greatly contributes to versatility of multimedia information sources having a strong resistance against errors on a transmission path and a stable transmission quality.
A Standard Definition Television (SDTV) system is widely used even in present days as a standard television system, which uses 525 scan lines in Japan and North America and 625 lines in Europe. However, recent digitalization of broadcasting requests for high image quality and large screen in displaying visual information, and a program is requested to be compatible with High Definition Television (HDTV). A transmission/reception system using a wireless camera is therefore expected to be compatible with HDTV.
The present inventor has proposed a data transmission system which can transmit high density information in conformity with HDTV specifications or the like even via a wireless transmission path having an arrow frequency band, without lowering a bit rate (refer to patent document 1: Japanese Patent Application Publication No. 2002-344965). A transmitter of the data transmission system described in patent document 1 divides an information code series (transport stream (TS)) in conformity with, for example, Moving Picture Experts Group (MPEG) 2-TS, in the unit of TS packet to form a plurality of divided transport streams. Each divided transport stream is expanded in time and modulated through Orthogonal Frequency Division Multiplexing (OFDM) and thereafter sent at the same time to different wireless transmission paths via different transmission units (high frequency amplifiers and transmission antennas). In correspondence to this, a receiver side prepares a number of reception units (high frequency amplifiers and frequency converter circuits) and receives separately information (divided transport streams) sent via different wireless transmission paths. At each reception channel, data of a received signal is OFDM-demodulated, and the transport stream having the original high bit rate is reproduced by executing time compression and multiplexing in a packet synchronization state.
In a present on-scene site such as live broadcasting and coverage/reporting on the spot, it is important to effectively use some use-permitted frequency bandwidth as much as possible. A specific operation example is given herein. If the number of necessary wireless cameras is determined already which are used for live broadcasting relaying of sports, events or large scale news sites and if all cameras are assigned to HDTV, a frequency bandwidth allocated to each wireless camera becomes narrow because of a limit in a usable frequency band, and the image quality is therefore lowered because of a small amount of information of a signal transmitted from each wireless camera. Conversely, if the image quality is to be retained, the number of wireless cameras usable in the limited frequency band becomes small and video and audio information necessary for producing a program cannot be collected sufficiently. The following operation setting may be used. Of several wireless cameras, about one or two HDTV cameras are allocated to the site where a main angle having a relatively long broadcasting time is shot. An video image shot at a special angle and at a special site, which is supposed to have a sporadic and short broadcasting time and often broadcast in a small screen, has a low priority order. Accordingly, it is conceivable that SDTV cameras are assigned to the low priority order shooting. For example, in sports live broadcasting, a live sport video has a high priority order, and a video of an announcer, a commentator, or a state of audience has a low priority order. At an on-scene site of live broadcasting and coverage, however, there is a possibility, for example, that a failed HDTV camera is replaced with a reserved SDTV camera or that a allocated position for an SDTV camera is changed to a site to be broadcast as a main video, because of an accidental event. There is also a case in which an HDTV camera and an SDTV camera are desired to be used swichably because of a request from program production.
In such cases, since the usable frequency band is fixed for HDTV cameras and SDTV cameras presently used, a change in a transmission rate in accordance with a state change during the operation described above, cannot be dealt with flexibly. From this reason, there is a high possibility of an improper screen quality for broadcasting images and program production, such as the case that a video with rough images in conformity with the SDTV specifications are forced to be broadcast for a long period irrespective of HDTV broadcasting or conversely the case that a video with high definition images in conformity with the HDTV specifications are used for a small screen display.
If the transmission technologies described in the above-cited Patent Document 1 are applied to wireless cameras, a broadband signal output from an HDTV camera can be transmitted by dividing it into narrowband channels similar to for an SDTV camera. It is therefore advantageous in that a high quality image can be transmitted at a high bit rate even if the frequency band is relatively narrow. However, since the number of wireless transmission paths (transmission channels) increases, a requisite for a wide frequency bandwidth still remains as a whole. Although the transmitter described in Patent Document 1 is structured by considering transmission of a high transmission rate signal such as an HDTV signal, it is not considered at all to output a relatively low transmission rate signal such as an SDTV signal from its receiver. Therefore, the transmitter described in Patent Document 1 cannot be used as a receiver of the above-described wireless transmission/reception system capable of flexible operation, although it can be used as a wireless transmitter dedicated to HDTV.